1. Field of the Invention
The present invention relates to a developing apparatus of photoresist suitable for use in fabrication of optical disks.
2. Description of the Related Art
In an optical disk, grooves are formed on both sides of its recording track and convex and concave pits are formed as information recorded thereon.
In fabricating such optical disk, a photoresist is applied to the surface of a glass substrate, for example, and the photoresist is exposed to pattern irradiation and then developed, and, thereby, convex and concave photoresist patterns corresponding to the grooves or pits are formed. Then, such a process as plating is performed on such convex and concave surface or on a convex and concave surface obtained by etching the surface of the substrate with the photoresist used as a mask and, thereby, a master for example of a stamper for forming the optical disk is fabricated.
In order to fabricate such photoresist patterns accurately, it is required in performing development of the photoresist after its exposure to irradiation to set up the developing time properly taking such conditions as temperature of the developing solution into consideration, so that neither over-development nor under-development is made and, thereby, desired patterns, i.e., grooves for the track or information pits being accurate in width, depth, and so on are formed.
Since the grooves in an optical disk or the like are arranged at an equal pitch in the radial direction of the optical disk and the information pits are also arranged substantially at an equal pitch in the radial direction under the condition of the substrate being rotated, equi-pitch windows are produced in the photoresist as development advances in the course of development of a latent image produced in the photoresist by its exposure to an irradiation corresponding to the pattern of grooves or the pattern of information pits. Making use of such equi-pitch windows as a diffraction grating, there is a method for controlling the development to be performed neither excessively nor deficiently, in which monitor light with a long wavelength to which the photoresist is not photosensitive is irradiated on such diffraction grating and the developing process is stopped according to results of detection of the light diffracted thereby.
The relative principle will be described with reference to FIG. 3. In this case, a photoresist layer 2 applied to the surface of a light transmitting substrate 1 such as a glass substrate is exposed to an irradiation having a required pattern for example of grooves or information pits.
Then, the photoresist layer 2 exposed to the pattern irradiation and having a latent image produced by the exposure is normally developed, for example by being subjected to a spray of a developing solution. In the course of the development, for example, monitor light L.sub.M is irradiated on the side where the photoresist layer 2 is formed, and the light of first-order diffracted light L.sub.M1, for example, produced by the equi-pitch patterns formed as the development of the photoresist layer 2 advances and serving as a diffraction grating, led out from the opposite side of the substrate 1 is detected by a photodetector 3 such as a photodiode. The monitor light L.sub.M is set to have a predetermined wavelength .lambda. and it is adapted such that, when desired equi-pitch patterns are formed, the output of the photodetector 3 reaches a predetermined value, and the developing process on the photoresist layer 2 is stopped by this detection signal, and thereby the development is performed neither excessively nor deficiently to provide developed patterns with a predetermined width and hence predetermined distance and depth.
In this case, as the monitor light L.sub.M, coherent light with a long wavelength not exposing the photoresist is used.
However, as the track pitch, for example, becomes smaller to meet the demand for higher recording density, the pitch of grooves or radial spacing between pit trains of the disk becomes smaller, that is, the pitch P of the photoresists on the optical disk shown in its section in the radial direction of FIG. 3, for example, becomes smaller. Then, there arises a problem that the diffracted light of the monitor light entering the light transmitting substrate 1 is not let out of the surface on the opposite side of the substrate toward the optical detector element but totally internally reflected at the surface.
Now, denoting, as shown in FIG. 3, the refractive index of the substrate 1 by n, the refractive index of the external media, for example the air, by n.sub.0, the angle of diffraction of the monitor light with a wavelength of .lambda. by .theta..sub.1, and the angle of emission by .theta..sub.2, a relationship given by the following expression (1) is obtained as to the angle of first-order diffraction .theta..sub.1 within the light transmitting substrate 1, i.e., the glass substrate 1, EQU sin.theta..sub.1 =n.sub.0 .lambda./nP. (1)
The relationship between the angle of diffraction .theta..sub.1 and the angle of emission .theta..sub.2 is expressed as EQU nsin.theta..sub.1 =n.sub.0 sin.theta..sub.2. (2)
Assuming now that n.sub.0 =1, and substituting the condition for the diffracted light to be totally reflected internally at the surface of the substrate 1, i.e., .theta..sub.2 .gtoreq..pi./2, for the above expressions (1) and (2), we obtain P=.lambda.. That is, when the pitch P of the patterns becomes smaller than the wavelength .lambda. of the monitor light L.sub.M, the monitoring according to the method of FIG. 3 becomes impossible.